It's possible to download SETI@HOME on several computers with the same account. I think this can explain the difference.
But there also may be a bug.
I sent my 4th unit yesterday evening.
I had a message "server problem". I clic on "retry" and did something else (I went to see the SETI League Team stats).
When I came back to SETI@HOME, I had received an other unit but my personal stat were not correctly updated (5 unit sent, 3 units received 101 hr), in my computer and also on the web.
I hope it's just the user info txt file that isn't updated and that they received my unit.